Magneto-resistance (MR) is the change of resistivity of certain materials in the presence of a magnetic field. This phenomenon is the principle governing the operation of most current magnetic read heads. MR can be significantly increased by means of a structure known as a spin valve or SV. The resulting increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are a low coercivity (free) ferromagnetic layer, a non-magnetic spacer layer, and a high coercivity ferromagnetic layer. The latter is usually formed out of a soft ferromagnetic layer that is pinned magnetically by a nearby layer of antiferromagnetic material. Alternatively, a synthetically pinned device (formed by sandwiching an antiferromagnetic coupling layer between two antiparallel ferromagnetic layers) may be used to replace the ferromagnetic pinned layer. This results in an increase in the size of the pinning field so that a more stable pinned layer is obtained. Also important for the construction of a successful spin valve is the insertion of a seed layer between the substrate and the above mentioned layers. The seed layer serves to influence the crystalline structure of these layers and hence their magnetic properties.
When the free layer is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field. If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers, suffer less scattering. Thus, the resistance at this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase.
Conventionally, spin valves in which the pinned layer is below the free layer are referred to as bottom spin valves (with top spin valves having the pinned layer above the free layer).
The change in resistance, dR (relative to its value R in the absence of a magnetic field) is referred to as the GMR ratio. Its value is typically between about 10 and 15% As magnetic read heads continue to shrink in area it becomes an ongoing challenge to be able to retain a GMR ratio of this order.
An important improvement in the technology was accomplished when it was found that seed layers of NiCr and/or NiFeCr led to significantly larger GMR ratios than did tantalum seeds, which had previously been in widespread use. However, this improvement came at a price. In particular, the coercivity (Hc) and the anisotropy field (Hk) of the free layer were found to now be much worse (larger) than with a Ta seed. From an application point of view, a free layer with large Hc and Hk is a problem because such a device will show lower sensitivity, bigger hysteresis and impact peak asymmetry as well as stability.
Hc and Hk can both be reduced by using an intrinsically magnetically soft ferromagnetic materials for the free layer. NiFe is a good example of such a material but other processing requirements require the free layer to go through high temperature annealing several times, during which episodes impurities (from the spacer layer) could diffuse into it. The solution to this problem that has been adopted by the prior art has been the insertion of a layer of CoFe between the NiFe layer and the spacer layer, this CoFe/NiFe bilayer now serving as the free layer. Since CoFe is more refractory than NiFe it acts as an effective diffusion barrier. However, it is also much harder (magnetically) than NiFe so the resulting Hc and Hk are not as low as desired. This can be seen in FIG. 1 which is a hysteresis curve for a CoFe layer 5 Å thick. Curve 11 was taken along the easy axis and curve 12 along the hard axis.
The present invention shows how a device having both high GMR ratio and low Hc and Hk can be formed.
A routine search of the prior art was performed with the following references of interest being found:
(1) Kanai et al. IEEE Trans Mag. MAG-32, 3368 (1996)
(2) Kanai et al. IEEE Trans Mag. MAG-33, 2872 (1997)
Ref. (1) teaches that a NiFe/CoFe bilayer gives good results as a free and/or pinned layer while ref. (2) shows that similar or better results can be obtained with a NiFe/CoFeX bilayer.
Other references of interest include U.S. Pat. No. 6,338,899 B1 (Fukuzawa et al.) which shows a free layer of CoFeB. In U.S. Pat. No. 6,101,072, Hayashi discloses a free layer of CoFeB. Gill, in U.S. Pat. No. 6,072,671 makes reference to work on NIFe/CoFeB SV heads while Chen et al. in U.S. Pat. No. 5,864,450, show a CoFeX material layer, but in a different context from the present invention.